High permeability metal casting plaster and method of making same

ABSTRACT

A high permeability metal casting plaster is used for production of high accuracy simulated castings for prototypes and any other plastermold castings. Methods are provided that create the high permeability metal casting plaster and mold. Also, a high permeability metal casting plaster is disclosed that uses control ground and screened alpha-hemihydrate gypsum, control ground and screened beta-hemihydrate gypsum, sucrose for protection against thermal shock, and a foaming agent.

This is a division of application Ser. No. 592,618, filed on Oct. 3,1990.

BACKGROUND OF THE INVENTION

The present invention relates to a high permeability casting plaster andmore particularly to a metal casting plaster of high permeability thatallows any plastermold castings and in particular the simulated diecasting of parts used primarily for the creation of prototype and shortrun production parts which will substantially replicate the appearanceand function of conventionally die cast parts.

It will be appreciated by those skilled in the art that the use ofgypsum-type materials in the manufacture of molds used for casting ofmetals is well known. The need for inexpensive simulated die casting isparticularly important for the creation of prototype or short runproduction parts which can be tested prior to the outlay of large sumsof money to develop a permanent steel die. To this end, there have beenseveral attempts to provide such a plaster for allowing the creation ofprototype and short run production parts.

One such attempt is disclosed by R. Reingen in U.S. Pat. No. 3,748,290.The Reingen patent briefly mentions the use of the addition of certainchemicals to a hemihydrate gypsum. The specific application of Reingenis directed toward ceramic molding. However, Reingen does mention thathis method has had certain success with the manufacture of other molds,patterns, and pressed pieces. However, the Reingen patent does notcreate the thin wall casting that is required to develop prototypes.

U.S. Pat. No. 4,749,431 issued to Fassle et al. on June 7, 1988,discloses the use of alpha and beta hemihydrates that are added to glassfibers. However, this disclosure, too, does not describe the simulateddie casting that is required to cast a prototype piece.

Several problems have plagued the aluminum/zinc alloy casting industryfor many years. For example, the wall thickness of the metal castingsvary due to irregularities in the surface of the mold. Also, turbulencein the molten metal caused by gases trapped in the mold cavity havecaused poor metal wall structure. Further, low strength mold wallsseparate and crack under high temperatures or pressure, resulting inflashings or fins on the metal walls of the casting. Conventionalcasting plasters, when cured, have also lacked the uniform permeabilitynecessary to provide uniform gas diffusion through and cooling of themolten metal during casting.

What is needed, then, is a high permeability metal casting plaster thancan be used inexpensively to create simulated die castings. This neededplaster must produce a harder and smoother skin on the plaster mold.This plaster must also hold the foam cells in the plaster mold inuniform suspension. To prevent large, irregular foam cells from formingdirectly behind the skin, which causes the collapse of the mold wallunder the weight of the molten alloy, the plaster must promote uniformdensity throughout the finished plaster mold. The plaster must alsocontrol cracking and micro-cracking of the mold surface that is causedby thermal expansion during drying and subsequent exposure to hightemperature by the molten alloy. A plaster having these qualities andcharacteristics is presently lacking in the art.

SUMMARY OF THE INVENTION

The present invention relates to the manufacture of simulated diecastings using a mold formed of permeable casting plaster. A mastermodel is produced from blueprints supplied by the customer having theshape, dimensions, and finish qualities of the desired cast metalend-product. A cavity or negative is then made by investing rubber andplastic over the master model. From the negative of the master model, apositive of the master model is then produced of rubber and plasticwhich is called a pattern tooling. The pattern tooling is then placed ina frame or box with novel highly permeable casting plaster of thepresent invention to form the mold.

The casting plaster slurry disclosed herein uses alpha-hemihydrateplaster that is screened to increase the percentage of small gypsumparticles in the plaster, thereby permitting more material to flow tothe outside of the mold to form a tighter skin. Beta-hemihydrate plasteris added to control the amount of alpha material that flows to the skin,thereby creating greater uniformity of foam cellular structurethroughout the mold. A foaming agent is added. Sucrose is added to themixture to protect the formed crystalline structure against thermalshock. Talc, Type 1 Portland cement, and an accelerator are also addedin the preferred embodiment to the dry mix that is added to water tocreate the slurry. During mixing and curing of the plaster, the water toplaster ratio and water temperature are controlled to achieve thedesired mold skin thickness, skin strength, and uniformity of cellularstructure.

DESCRIPTION OF THE PREFERRED EMBODIMENT

At the request of a customer, a master model of the desired part to bedie cast is produced from wood, metal, or plastic. The dimensions of themaster model are taken from a blueprint, drawing, or sketch provided bythe customer. The master model must duplicate all of the importantcharacteristics needed in the actual production die casting--appearance,dimensional accuracy, and mechanical and chemical properties. Becausethe simulated die castings are used for prototype purposes, the mastermodel may go through many engineering changes and redesigns before finalapproval. The word "simulated" is used to differentiate between castingsproduced in a plastermold and those produced in a steel mold. Only acasting produced in a steel mold can be called a die casting.

Upon final approval by the customer, a duplicate of the master model isproduced in conventional fashion using a variety of materials, primarilysynthetic plastics and rubbers, to replicate the original master model.Rubber and plastic is invested over the master model in order to developa cavity or negative of the master model. Additional rubber and plasticis then introduced into the negative cavity to produce a positive of theoriginal master model or "pattern tooling".

The pattern tooling is then placed in a frame or box of wood, metal, orother suitable material capable of containing a slurry of highlypermeable plaster. Various types of plaster (calcium sulphate) areblended with certain other ingredients to form a dry material which isthen combined with heated free water. This mixture is allowed to becomesaturated and is then mixed for a specific period of time until asatisfactory slurry is produced. The water to plaster ratio and watertemperature are critical to achieve the desired skin thickness, skinstrength, and uniform cellular structure of the mold. In the preferredembodiment, the water to plaster ratio is substantially 1:1. The watertemperature is maintained at approximately 95° F. to 110° F.

The slurry is then poured over the pattern tooling and allowed toharden. During this process, a heavier sediment in the slurry settles tothe face of the pattern tooling and forms a skin which duplicates theexact detail and finish of the pattern tooling. Directly behind theskin, cells are created, much like those of a sponge. These cells givethe plaster mold its high permeability. The plaster of the presentinvention promotes uniformity in the size and distribution of foamcells, which improves the mechanical properties, surface appearance anddimensional accuracy of the final simulated casting. After the plasterslurry hardens sufficiently, the plaster mold is then removed from thepattern tooling and is placed in a drying oven to remove the free wateradded during the original preparation of the plaster slurry. The set,dry mold is a dihydrate (gypsum) containing two molecules of water inthe crystal. It is surface calcined when the mold wall comes in contactwith the molten metal, and 1.5 to 2 molecules of water are released andforced through the mold wall, into the cellular structure of the moldbody. This action maintains the smooth metal surface by eliminating thegas (steam) in the direction of least resistance.

The plaster mold, after having been thoroughly dried to less than onepercent moisture content, is now prepared to receive the molten aluminumand/or zinc alloys. The molten metal is conventionally introducedthrough a sprue or opening of the mold into the mold cavity, usingeither gravity, pressure, vacuum, or combinations thereof. The formationof the casting is complete upon solidification of the metal within thecavity. The casting is removed from the plaster mold, at which time thecasting is trimmed and ready for shipment or subsequent secondarymachining operations.

The novel plaster of the present invention starts with analpha-hemihydrate gypsum as part of the dry mix. This alpha-hemihydrategypsum is a calcined gypsum material where 1.5 to 2 water molecules havebeen removed from the crystals during calcination. Examples of thisalpha-hemihydrate gypsum are the conventional gypsum product sold by USGas "White Hydrocal®" and sold by Georgia Pacific as "Densecal WH®". Thisalpha-hemihydrate gypsum is screened to obtain a finer product. Thenormal consistency of this alpha-hemihydrate gypsum as tested per ASTM472-79, Methods 5 and 6, has 30 to 32 cc water per 100 gm of plaster.The plunger penetration of this alpha-hemihydrate gypsum is 30 mm, plusor minus 2 mm. In the preferred embodiment, the alpha-hemihydrate gypsumyields the following alcohol wash sieve analysis as per the appendix toASTM 472-79:

    ______________________________________                                        Sieve size (in microns)   Percent                                             ______________________________________                                        Through 30 mesh                                                                            (-590 micron)                                                                              100%    average                                     Through 50 mesh                                                                            (-297 micron)                                                                              99.99   average                                     Through 100 mesh                                                                           (-149 micron)                                                                              99.60   average                                     Through 200 mesh                                                                           (-74 micron) 97.60   average                                     Through 325 mesh                                                                           (-44 micron) 97.30   average                                     ______________________________________                                    

By screening and the controlled grinding of the alpha-hemihydrategypsum, the percentage of small gypsum particles in the plaster isincreased, thereby permitting more material to flow to the outside ofthe mold to form a tighter skin. Prepared as described, thealpha-hemihydrate should have a particle size distribution that is 4 to5% finer in the 0.5 to 44 micron range.

Beta-hemihydrate is then added to the dry mix to control the amount ofalpha-hemihydrate that flows to the skin after saturation. Thisbeta-hemihydrate gives greater uniformity of foam cellular structurethroughout the mold. The beta-hemihydrate gypsum is atmosphericallycalcined and reground to meet certain specifications. In the preferredembodiment, the water ratio as per ASTM 472-79, Methods 5 and 6, is 48to 52 cc per 100 grams of plaster. The plunger penetration is 30 mm plusor minus 2 mm. An alcohol wash sieve analysis as per the appendix toASTM 472-79 yields the following results:

    ______________________________________                                        Sieve size (in microns)                                                                            Percent                                                  ______________________________________                                        Through 50 mesh                                                                            (-297 microns)                                                                            99.0% average                                        Through 100 mesh                                                                           (-149 microns)                                                                            99.0% average                                        Through 200 mesh                                                                           (-74 microns)                                                                             85.0% average                                        Through 325 mesh                                                                           (-44 microns)                                                                             70.0% average                                        ______________________________________                                    

A talc is also added to the premix. In the preferred embodiment, #2882282 Tex-Ceram Talc® CAS No. 14807-96-6 is used.

Type 1 Portland cement is added to the premix. When the tricalciumaluminate concentrate of the Portland Cement is 8% or less, the amountof sulphur trioxide should not exceed 2.5%. In the preferred embodiment,the product is to be free of all package lumps and screened through a 30mesh screen.

An anionic biodegradable foaming agent, such as alkyl naphthalenesuphonate, is added. In the preferred embodiment, Petro BAF® is used.

In the preferred embodiment, the premix that is produced has 80%alpha-hemihydrate and 20% Petro BAF® foaming agent. This premix blend iskept in a closed container and should not be stored for more than 14days.

Granulated sugar or sucrose is added as a colloidal retarder in thepreferred embodiment to the mixture to protect the formed crystallinestructure against thermal shock. This additive lowers the solubilityrate of calcium sulphate by coating some of the grains of calcinedgypsum.

Dihydrate gypsum which is freshly ground is used in the preferredembodiment of the plaster as an accelerator. The accelerator is screenedthrough a 30 mesh. It is kept in a closed container. In the preferredembodiment, any Terra Alba-type gypsum may be used and is produced bymixing regular beta-hemihydrate plaster with water at a ratio of 70-80parts water to 100 parts plaster by weight. The water/beta-hemihydrateplaster is then set and dried. The dry cast is then micropulverized to apowder passing through a 30 mesh screen.

A preferred powder mixture of these various materials in the preferredembodiment of the plaster is as follows:

    ______________________________________                                        Materials         Percent  Pounds/Ton                                         ______________________________________                                        Alpha-hemihydrate 51.450   1029.0                                             Beta-hemihydrate  20.000   400.0                                              Talc              25.000   500.0                                              Portland cement Type 1                                                                           1.000   20.0                                               Premix foaming agent                                                                             1.135   27.5                                               Sucrose            0.050    1.0                                               Accelerator        1.125   22.5                                               ______________________________________                                    

The uniformly blended dry powder is added to free water heated toapproximately 95° to 105° F., at a mix of 1:1 (100 cc per 100 gm). Themixture is allowed to soak for one minute, saturating the powder, and isthen mixed for approximately one minute with a high speed mixer. Thesoaking time is to be measured from the moment the powder is added tothe water.

When the dissolved plaster is prepared as described, the foam cellsproduced should have a diameter in the range of 2 to 4 mm. Also, theamorphous structure of the beta-hemihydrate particle will hold the cellsin a more uniform suspension. It is desired that the slurry produced bysoaking and mixing the powder with heated water have a volume increaseduring mixing of approximately 90% to 100% in a Pat diameter. Also, thepowder, when mixed with water at a ratio of 100 gm/56 cc should producea Pat minimum diameter of 4.0" with a maximum of 5". The Pat diameter ismeasured by soaking the powder and water for 1 minute, hand mixing for 1minute, and pouring the mixture from the container onto a clear plateglass from a height of 2". The preferred setting time for the preferredembodiment is approximately 12 to 15 minutes.

Thus, although there have been described particular embodiments of thepresent invention of a high permeability metal casting plaster, it isnot intended that such references be construed as limitations upon thescope of this invention except as set forth in the following claims.Further, although there have been described certain specifications usedin the preferred embodiment, it is not intended that such be construedas limitations upon the scope of this invention, except as set forth inthe following claims.

What I claim is:
 1. A method of producing a high permeability castingplaster comprising the steps:(a) making a premix of screenedalpha-hemihydrate gypsum and a foaming agent; (b) blending said premixwith beta-hemihydrate gypsum in a ratio of approximately 2.5 to 1 byweight of alpha- to beta-hemihydrate gypsums; (c) adding talc, Portlandcement, a thermal shock additive, and an accelerator to said blend,thereby forming a blended dry powder having a Pat diameter ofapproximately 4.0" to 5.0"; and (d) combining said powder with heatedwater at a ratio of approximately 1 to 1, soaking said powder untilsaturated; and (e) mixing said combination of said powder and wateruntil a slurry is formed which, during hardening, will have a volumeincrease of approximately 90% to 100% and a setting time ofapproximately 15 minutes; wherein said alpha-hemihydrate gypsumcomprising dry particles such that an average of 100% of said particleshave a diameter of no more than 590 microns, and such that at least anaverage of 97.3% of said particles have a diameter of no more than 44microns; said beta-hemihydrate gypsum comprising dry particles such thatan average of 99% of said particles have a diameter of no more than 297microns, and at least an average of 70% of said particles have adiameter of no more than 44 microns.
 2. The method of claim 1 whereinthe proportions by weight of each component of said powder is asfollows: substantially 51.45% of alpha-hemihydrate, substantially 20% ofbeta-hemihydrate, substantially 25% of talc, substantially 1% ofPortland cement, substantially 1.135% of foaming agent, substantially0.05% of thermal shock additive, and substantially 1.25% of saidaccelerator.
 3. A plaster metal casting mold made by the process ofblending a powder of screened alpha-hemihydrate gypsum withbeta-hemihydrate gypsum, talc, Portland cement, foaming agent, thermalshock additive, and an accelerator, mixing said powder with heated waterat an approximate ratio of 1 to 1 forming a slurry, and pouring saidslurry over a pattern tooling; wherein said alpha-hemihydrate gypsumcomprising dry particles such that an average of 100% of said particleshave a diameter of no more than 590 microns, and such that at least anaverage of 97.3% of said particles have a diameter of no more than 44microns; said beta-hemihydrate gypsum comprising dry particles such thatan average of 99% of said particles have a diameter of no more than 297microns, and at least an average of 70% of said particles have adiameter of no more than 44 microns.
 4. A plaster metal casting moldmade by the process of:(a) making a premix of screened alpha-hemihydrategypsum and a foaming agent; (b) blending said premix withbeta-hemihydrate gypsum in a ratio of approximately 2.5 to 1 by weightof alpha- to beta-hemihydrate gypsums; wherein said alpha-hemihydrategypsum comprising dry particles such that an average of 100% of saidparticles have a diameter of no more than 590 microns, and such that atleast an average of 97.3% of said particles have a diameter of no morethan 44 microns; said beta-hemihydrate gypsum comprising dry particlessuch that an average of 99% of said particles have a diameter of no morethan 297 microns, and at least an average of 70% of said particles havea diameter of no more than 44 microns; (c) adding talc, Portland cement,a thermal shock additive, and an accelerator to said blend, therebyforming a blended dry powder having a Pat diameter of approximately 4,.0to 5.0"; (d) combining said powder with heated water at a ratio ofapproximately 1 to 1, soaking said powder until saturated, and mixingsaid combination of said powder and water until a slurry is formedwhich, during hardening, will have a volume increase of approximately90% to 100% and a setting time of approximately 15 minutes; and (e)pouring said slurry over a pattern tooling and allowing it to harden. 5.A method of producing a high permeability casting plaster comprising thesteps of:(a) making a premix of control ground alpha-hemihydrate gypsumand a foaming agent; (b) blending said premix with beta-hemihydrategypsum in the ratio of approximately 2.5 to 1 by weight of alpha- tobeta-hemihydrate gypsum; (c) adding talc, Portland cement, a thermalshock additive, and an accelerator to said blend, thereby forming ablended dry powder having a Pat diameter of approximately 4.0 inches to5.0 inches; and (d) combining said powder with heated water at a ratioof approximately I to 1, soaking said powder until saturated; and (e)mixing said combination of said powder and water until a slurry isformed which, during hardening, will have a volume increase ofapproximately 90% to 100% and a setting time of approximately 15minutes; wherein said alpha-hemihydrate gypsum comprising dry particlessuch that an average of 100% of said particles have a diameter of nomore than 590 microns, and such that at least an average of 97.3% ofsaid particles have a diameter of no more than 44 microns; saidbeta-hemihydrate gypsum comprising dry particles such that an average of99% of said particles have a diameter of no more than 297 microns, andat least an average of 70% of said particles have a diameter of no morethan 44 microns.